Water heater with heat insulating coating on tubes



Aug. 3i, H965 A. L. MILLER 3,293,404

WATER HEATER WITH HEAT INSULATING COATING ON TUBES Filed Feb. 17. 1961 "burners and interfere with proper combustion.

eral, the presence of condensate on the water tubes in The present invention relates generally to Water heaters and is more particularly exemplified in such devices of the continuous ilow type.

Continous ilow water heaters are those in which water 1s heated only during the supply thereof, as opposed to storage type heaters in which the water contained in the storage tank is maintained at the desired supply temperature and the water is re-circulated to bring the temperature up to the desired hot water temperature whenever a portion of the contents of the storage tank is withdrawn. One example of a specific use of a continuous flow water heater is in heating water for a swimming pool, although it is of obvious utility for the supply of hot water generally.

One of the disadvantages encountered in the use of continuous flow water heaters, particularly those which are gas and oil fueled, is the fact that the input end of the water tube is, due to the incoming cold water, constantly at a temperature considerably below the dew point of the combustion products surrounding the outside of the tube. This results in the formation of condensate on the outer surface of the tube along a substantial length thereof due to the fact that the products of combustion are heavily saturated with Water vapor and upon being cooled to the temperature of the inflowing water, con` dense out most of their moisture. This condensate ini cludes, in solution, highly corrosive combustion products such as sulfurous acid and, in suspension, soot and other solid materials. This highly corrosive condensate not only fouls the exterior of the tubes, but drips down on the burners to corrode the same and deposit soot and other solid materials which have a tendency to plug the In gensufticient quantity to drip down on the burners of they heater or the lining of the re box creates serious corrosion problems.

Various proposals have been advanced to solve the above problem by raising the skin temperature of the water tubes above the dew pointl of the water vapor in the combustion products thus to prevent the formation of condensate. These solutions have in general involved either moving parts, with attendant increase in cost and complexity, or the heating of water to a relatively high temperature to reduce the length of the cold portion of the heater tubes wherein the surface temperature is below the dew point -of the flue gases. This latter expedient only substitutes another diiculty however, i.e. the deposit of minerals on the interior of the heater tubes. The deposit of such minerals from so-called hard Waters v increases as the temperature of the water increases.

According to the present invention the exterior surface temperature of the Water tubes is increased above the dew point of the iiue gases by coating the tube or tubes with a heat insulating coating. Such coating may be, for example, a phenolic or epoxy resin which may have included therein an admixture of carbon black, powdered mica or similar material to increase its ab- Y in the description hereinafter.

With the foregoing in mind an object of the present United States Patent ICC ` invention is to provide an improved water heater which eliminates condensation of moisture on the exterior of heat exchange tubes.

Another object of this invention is to provide an irnproved water heater of the water tube type which elimininates the corrosion and burner plugging problems arising from the formation of condensate on the exterior surface of the water tubes.

Another object of this invention is the provision of an improved Water heater employing heat exchange tubes through which water to be heated flows and about which pass products of combustion, in which the exterior surface temperature of the heat exchange tubes is raised above the dew po-int of the flue gases to eliminate the condensation of moisture on the exterior of the tubes.

A further object of the present invention is to provide a water heater employing heat exchange water tubes about which pass products of combustion, with a heat insulating coating for said tubes which maintains the exterior surface temperature of the coating above the dew point of the flue gases.

A still further object of this invention is to provide a water heater of the heat exchange tube type with heat insulating coat-ings over at least the portions of the tubes which would otherwise have an exterior operating surface temperature below the dew point of the ilue gases, the coatings having exterior surfaces with temperatures above the dew point of the ilue gases.

Yet another object of this invention is to provide a water heater of the heat exchange tube type with heat insulating coatings for said tubes to maintain the temperature of the surfaces contacting the flue gases above the dew point temperature thereof and in which the coating includes an admixture to increase the absorption of radiant energy thereby.

These and additional objects and features of the present invention will be readily apparent to those skilled in the art from the following detailed description of a presently preferred embodiment thereof and the appended drawings in which:

FIGURE 1 is a partially cut away, perspective view of i a continuous ow water heater embodying the present invention.

FIGURE. 2 is a detailed View of a heat exchange tube for the Water heater showing a heat insulating coating thereon.

FlGURE 3 is a view similar to FIGURE 2 and showing the coating as including an admixture of carbon black or powdered mica to increase the absorption of radiant energy.

With the exception of the coating on the sheet exchange tubes, the water heater of the present invention may be of conventional mechanical form including a re box 11, an outer covering 12 and an insulating layer of rock Wool or the like 13, therebetween. In the bottom of the fire box 11 is disposed a series of burners 14, fed from a fuel line 15 through a control valve 16, a pressure regulator 17 and a solenoid control 13. A conventional safety pilot light is indicated at 19.

The water heater shown by way of example in FIGURE l is of the multiple water tube, single pass type employing A an input manifold 21 provided with an internally threaded sorption of radiant energy. Such coatings may be apv plied over only those portions of the Water heater tubes pipe connection 22 to receive the input pipe, not shown. At the rear of the fire box is mounted an outlet manifold 23 having an outlet pipe 24 leading therefrom. Between the manifolds 21 and 23 of the burners 14 extend parallel heat exchange tubes 25, within which the Water to be heated flows in single, parallel passes from the inlet manifold 21 to the outlet manifold 23. The products of combustion from the burners 14 pass upwardly around the exterior surfaces of the heat exchange tubes 2S. An interior hood 26 of sheet metal serves to collect the 3 flue gases after they have passed in contact with the heat exchange tubes and directs such gases to the ue 27 at the top of the housing 12.

One of the tubes 25 of FIGURE 1 is shown in detail in FIGURE 2 as comprising an interior pipe 28, of good heat conducting material such as copper, upon which is disposed a heat resistant, heat insulating coating cornprised, for example, of a resinous material of the phenolic or epoxy type. The coating 29 may be applied uniformly along the tubes 25, or may be disposed in varying thickness diminishing from the inlet ends of the tubes 25 toward the outlet ends thereof in conformance with the increase in temperature of the water within the tubes. The purpose of the heat insulating coating 29 is to reduce the rate of heat transfer somewhat whereby to maintain the exterior surface of the coated heat exchange tube above the dew point of the flue gases so that condensate does not form thereon.

With or without diminution in the thickness thereof, the coating 29 may be applied over only those portions of the tubes 25 which would otherwise have surface temperatures below the dew point of the flue gases. Thus, the coating 29 may extend only from the inlet header 21 along the heat conducting tubes 28 to a position where (due to the heating of water owing therethrough) the temperature of the surfaces of the tubes 28 is above the dew point of the flue gases. Rrom this point on to the outlet header 23, the coating may be omitted from the tubes 25 in order to secure a more rapid heat exchange between the flue gases and the water within the tubes.

In certain instances, where the flue gases themselves are particularly corrosive, it may be desirable to run the coating 29 across the tubes 2S for their entire length between the headers 21 and 23 so that the surfaces of the heat conducting tubes will be protected from corrosion over their entire length.

While a single pass water heater has been shown in FIGURE l in which the water makes only one pass across the fire box of the heater, it is conventional to run the water in a multiplicity of passes back and forth across the fire box, and in this case it will be understood that the coating 29 may be placed on all the tube surfaces, only on the inlet pass of the tubes or only on those portions of the tubes, regardless of the direction of the pass, which would otherwise have a surface temperature below the dew point temperature of the ue gases.

In applying the resin coating 29 to the tubes 28, the outer surfaces of the tubes are cleaned to remove all contamination, especially oil or grease. A rough surface is then formed on the exterior of the tubes 28 (or such portion as is to receive a coating) by Sandblasting or by etching in acid. Finally, a resinous coating of phenolic or epoxy resin is applied to the outer surface of the tubes as, for example, by dipping or spraying, and the coating is conventionally heat-treated or chemically treated to form the finished heat resistant, heat insulating coating on the heat conducting tubes 2S, thus completing the finished heat exchange tubes 25.

Another feature of this invention is shown in FIGURE 3, in which a heat resistant and heat insulating coating 31 has admixed therein carbon black, powdered mica or comparable material to increase the absorption of radiant energy by the coating. In this form of the invention, the increase in surface temperature of the coated heat exchange tubes 25 is due not only to the heat insulating properties of the coating (as in the coating 29) but also is due in part to the increased absorption of radiant energy brought about by the incorporation in the basic resin (phenolic, epoxy or other) of the carbon black, powdered mica or comparable absorption increasing material.

With the coating of FIGURE 3, the corrosion and fouling of heat exchange plates and the burners and other parts of the water heater is eliminated, and at the same time an increased absorption of radiant energy is effected through the incorporation of the admixing materials with the basic resin coating. These effects are mutually modifying in that the increased absorption of radiant energy coacts with the insulating properties of the coating to effect a resultant increase in the outside surface temperature of the coated heat exchange tubes.

While certain preferred embodiments of the invention have been specifically shown and described, it will be understood that the invention is not limited thereto as many variations will be apparent to those skilled in the art and the invention is to be given its broadest interpretation within the terms of the following claims.

I claim:

1. A continuous flow water heater comprising: an input chamber having means to receive water to be heated; an outlet chamber having means to deliver heated water from the heater; a plurality of substantially parallel, generally horizontally disposed heat exchange water tubes intercommunicating said chambers; means to pass water-vapor laden flue gases in contact with the exterior of said tubes, said tubes receiving water from said input means at a temperature below the dew point of said gases and delivering heated water as a liquid to said outlet means; and a heat resistant and heat insulating coating on the outer surfaces of said tubes at least between said input chamber and a point along the length of the tubes at which their operating surface temperature is above the dew point of said gases, said coating having sufficient insulating properties to maintain the temperature at its exterior surface above said dew point and decreasing in thickness from said input chamber along the length of the tubes in accordance with the rise in temperature of the water passing through the tubes.

2. A continuous flow water heater comprising: an input chamber having means to receive water to be heated; an outlet chamber having means to deliver heated water from the heater; a plurality of substantially parallel generally horizontally disposed heat exchange tubes intercommunicating said chambers; means to pass water-vapor laden flue gases in contact with the exterior of said tubes, said tubes receiving water from said input means at a temperature below the dew point of said gases and delivering heated water as a liquid to said outlet means; and a heat resistant and heat insulating coating on the outer surfaces of said tubes at least between said input chamber and a point along the length of the tubes at which their operating surface temperature is above the dew point of said gases, said coating decreasing in thickness from said input chamber along the len-gth of the tubes in accordance with the rise in temperature of the water passing through the tubes while retaining a thickness suficient to maintain the exterior surface of the coating at a temperature above said dew point, said coating having admixed therein a finely divided material increasing its heat absorptive properties and raising its surface temperature.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Skiest, I.: Epoxy Resins, Reinhold Publishing Corporation, New York, N.Y., 1958.

Gould, D. F.: Phenolic Resins, Reinhold Publishing Corporation, New York, N.Y., 1959 (p. relied on).

PERCY L. PATRICK, Primary Examiner.

FREDERICK L. MATTESON,

OLEARY Examiners.

IR., ROBERT A. 

1. A CONTINUOUS FLOW WATER HEATER COMPRISING: AN INPUT CHAMBER HAVING MEANS TO RECEIVE WATER TO BE HEATED; AN OUTLET CHAMBER HAVING MEANS DELIVER HEATED WATER FROM THE HEATER; A PLURALITY OF SUBSTANTIALLY PARALLEL, GENERALLY HORIZONTALLY DISPOSED HEAT EXCHANGE WATER TUBES INTERCOMMUNICATING SAID CHAMBERS; MEANS TO PASS WATER-VAPOR LADEN FLUE GASES IN CONTACT WITH THE EXTERIOR OF SAID TUBES, SAID TUBES RECEIVING WATER FROM SAID INPUT MEANS AT A TEMPERATURE BELOW THE DEW POINT OF SAID GASES AND DELIVERING HEATED WATER AS A LIQUID TO SAID OUTLET MEANS; AND A HEAT RESISTANT AND HEAT INSULATING COATING ON THE OUTER SURFACES OF SAID TUBES AT LEAST BETWEEN SAID INPUT CHAMBER AND A POINT ALONG THE LENGTH OF THE TUBES AT WHICH THEIR OPERATING SURFACE TEMPERATURE IS ABOVE THE DEW POINT OF SAID GASES, SAID COATING HAVING SUFFICIENT INSULATING PROPERTIES TO MAINTAIN THE TEMPERATURE AT ITS EXTERIOR SURFACE ABOVE SAID DEW POINT AND DECREASING IN THICKNESS FROM SAID INPUT CHAMBER ALONG THE LENGTH OF THE TUBES IN ACCORDANCE WITH THE RISE IN TEMPERATURE OF THE WATER PASSING THROUGH THE TUBES. 